The present invention relates to the characterisation and optionally the automatic sorting of objects, in particular recyclable domestic packagings, by their constituent materials and/or by their colour, the combination of a material or of a constituent substance and of a colour hereinafter being called a category.
It relates to a device and a method for automatically inspecting travelling objects with characterisation and discrimination according to their chemical composition.
The machine according to the invention is particularly but not exclusively suitable for inspection purposes and optionally for sorting, at high speed, various recyclable plastics packagings, in particular bottles made of PET, HDPE, PVC, PP and PS, as well as paper/cardboard, composite (drink packs) or metal packagings.
However, this machine may also be used for inspecting and discriminating any other objects or articles containing organic chemical compounds and travelling with a substantially single layer planar presentation such as, for example, fruits (discrimination by sugar content), and discrimination may be carried out on the basis of a major or minor chemical compound or of a plurality of chemical compounds.
In addition, said discrimination may end with separation of the flow of objects by sorting in categories or merely with counting and characterisation of said flow.
There are already numerous machines and numerous methods of the aforementioned type, in particular for sorting packagings according to their constituent material.
However, these known machines all have fairly serious drawbacks and significant limitations.
Therefore, the sorting of domestic packagings is still largely manual at present, particularly in European countries where sorting by material is demanded by the authorities responsible for recycling but also in other countries.
Significant automation of sorting has recently taken place in Germany, but in a very particular context, at least with respect to plastics materials. Sorting criteria do not concern the material but the shape (films, hollow bodies, or various mixed plastics). These existing machines therefore sort a “mixed plastics” category from papers/cardboards, after aeraulic presorting of the films and manual presorting of hollow bodies. Machines for the sorting of composite packagings or metal packagings are also found.
Existing machines differ greatly in terms of efficacy, depending on the type of mechanical preparation of the flow of objects to be sorted. Three main solutions may be distinguished:                complete individualisation with a single object per receptacle, without grasping an object;        a thread-form flow, the objects being aligned one behind the other;        a planar flow, the objects being spread in bulk over a mat which is much larger than their largest dimension and being distributed in a single layer.        
Only the last solution has proven suitable, from the points of view of efficacy and productivity, for products which are as heterogeneous as refuse, in particular domestic refuse. In fact:                Complete individualisation has never been industrially proven. The prototypes developed with this type of presentation all ceased operating afterwards.        The thread-type flow already existed in industrial over-sorting machines in which the main flow was uniform and over-sorting involved removing a small percentage of undesirable objects. Applied to a heterogeneous flow of packagings, these thread-type systems operated on particularly clean flows. However, these machines have a limited throughput and necessitate the presence of manual operators upstream of the machine to remove objects likely to disturb operation, in particular large sheets of plastic and large containers. Therefore, they do not constitute a satisfactory solution for automation of sorting and have had little success.        Planar flows, on the other hand, have proven themselves as this is exactly the presentation of objects found in manual sorting. It is thus known how to carry it out simply in the context of domestic refuse, and the machines using this type of flow are suitable for bulk sorting conditions and have met with much greater success than the two other aforementioned types.        
Only planar flow sorting, involving the currently most effective machines, will therefore be discussed hereinafter.
The document EP-A-0 706 838 in the name of the Applicant presents a sorting machine and method suitable for objects in a planar flow. This machine uses at least one artificial vision system to locate the objects and to recognise their shape and their colour, a robotic arm to grasp and handle the objects and at least one complementary sensor to recognise their constituent material. This complementary sensor is advantageously an infrared spectrometer.
This system has the advantage of being basically a multimaterial system since the main packagings are sorted by material and/or by colour and are distributed in a plurality of suitable containers. The same machine may therefore sort up to eight different categories. Furthermore, the individual gripping of the objects guarantees an excellent quality of sorting, typically one error per 1,000 sorted objects.
However, the sorting rate of this system is limited by the individual gripping of the sorted objects and does not exceed 60 to 100 kg/h per sorting module. The only way to increase this rate is to cascade a plurality of identical sorting modules, and this increases the overall bulk of the machine and its cost.
The document U.S. Pat. No. 5,260,576 presents a planar sorting machine which emits overhead the flow of electromagnetic radiation received by transmission below the flow of objects. The intensity of this radiation enables the materials to be distinguished according to their relative opacity in transmission. Thus, if the radiation consists of X-rays, this document mentions satisfactory separation of PVC which contains an atom of chlorine which is opaque to X-rays, in comparison with the other plastics which do not contain any, in particular PET. Depending on the result, a row of nozzles will or will not eject one of the classes of objects downwards.
However, this detection principle is too basic for complex cases: all objects have a degree of opacity, and it will be appreciated that multiple thicknesses of a material which is only slightly opaque (for example PET/polyethylene terephthalate) may not be distinguished from a single thickness of a more opaque different material (for example, PVC—polyvinyl chloride). There is therefore the risk of ejecting all these sparingly opaque objects at once in error. In addition, this system can only distinguish PVC from other plastics: it is incapable of distinguishing PET from HDPE (high density polyethylene) or PAN (polyacylonitrile). Existing machines according to this document have limited efficacy and low outputs (proportions of desired objects from among the ejected objects): of 10 to 30%. Finally, a significant drawback of the transmission assembly is that at least one of the two elements, the sensor or the transmitter, has to be below the flow. There is therefore a risk of recurrent soiling or blockage of the lower element, necessitating repeated interventions at relatively short intervals.
The document EP-A-0 776 257 describes a planar sorting machine which has a high throughput and is capable of recognising one material from a plurality of materials. The material to be recognised is selected at the time of construction of the machine by appropriate fixed calibration.
In this machine, mere infrared lighting is emitted overhead and the sensor is also placed on top, so it analyses the light which is scattered back vertically by the objects.
Reception is effected via a plane or semicircular concave mirror extending over the entire width of the mat, then by a polygonal rotating mirror. The point of measurement is therefore scanned cyclically over the entire width of the mat.
The light received from the measuring point is then divided by an assembly of semi-reflective mirrors in a plurality of flows. Each flow passes through an interferential filter centred over a specific wavelength, then ends at a detector. Each detector therefore measures the proportion of received light contained in the bandwidth of the filter. Analysis of the relative intensities measured by the various detectors allows a decision as to whether the material present at the point of measurement is or is not the desired material. The number of filters mentioned in this document is between 3 and 6.
The presence of a large-sized mirror of this type constitutes a fragile point of the overall structure, elongates the detection/ejection distance, increases the overall bulk of the detection station and is likely to lead to distortion and introduce inhomogeneities in the light flux recovered for analysis, leading to errors of detection.
In such architecture, the speed of detection is the main issue: there are 25 to 50 measuring zones per line, and 100 to 150 lines have to be analysed per second in view of the speed of circulation of the flow. The magnitude is therefore 5,000 measurements/s. Such a speed involves significant constraints:                the detection algorithm must be sufficiently simple (therefore few operations and simple processing) to be carried out in real time;        the reception electronics must be very fast;        the quantity of light received must be sufficient in a very short time.        
The detection algorithm has to carry out two-dimensional reconstitution of the objects to be sorted before proceeding to eject them, and this necessitates a relatively large distance between the detection zone and the ejection zone, increasing the risks of erroneous ejection owing to a movement of the objects between detection and ejection.
The aforementioned problem concerning the quantity of light is critical and explains why the machine according to this document can only recognise a predefined material:                multimaterial recognition would necessitate the use of at least 8 to 16 wavelength ranges (or PLO) and not just 3 to 6 ranges;        in addition, the widths of the PLOs, which are relatively large in the example mentioned (32 to 114 nm) would have to be reduced in a range of 5 to 20 nm since a larger number of PLOs has to be distinguished in the same spectral width.        
The two effects are added together: the greatest number of PLOs would divide the quantity of light received by each filter by approximately 3; the reduced width of each PLO means that each filter would allow a fraction, which is about 5 times smaller, of the received light to pass through. To maintain the same level of signal, the lighting power required for the machine would therefore pass from one to 3×5=15 kW. Such a power would not be realistic (cost, energy consumption, heating).
The document WO 99/26734 presents a planar sorting machine having a high throughput, with architecture which is fairly close to the previous document but discloses multimaterial recognition.
To achieve this, this document approaches the problem of the quantity of light differently: it proposes a vision system upstream on the conveyor of infrared detection, this system being quite comparable to the one mentioned in the aforementioned document EP-A-0 706 838. This system allows each object present to be located and, in the region of infrared detection, allows a single measuring point which follows the travelling object to be controlled by a set of position-sensitive mirrors. The analysis time available becomes relatively long, of the order of 3 to 10 ms, as a single point is analysed per object. Implementation, although not specified, may therefore use known technology which is compatible with this analysis time. For example, a spectrometer with a bank of photodetectors (typically 256 components, each corresponding to a wavelength) with resolution of 4 to 6 nm per detector may be used.
However, this solution also has several drawbacks:                it necessitates additional material, namely a vision system;        it is dependent on the selection by vision of the point of spectrometric measurement on the object, and this may be awkward in the presence of labels or soiling;        it is dependent on the immobility of the object on the mat: as the two detections are made on zones of about 1 m×1 m, the object moves by at least 1 m between its detection by vision and its detection by spectrometry, then by 0.5 m on average between its detection by spectrometry and its final ejection. Immobility is never ensured when the conveyor advances at 2.5 m/s, particularly if the objects are bottles which are likely to roll.        
The machine described in this document is obviously more flexible but more expensive and much less effective than the previous one.
Finally, the document DE-A-1 96 09 916 describes a miniaturised spectrometer for a planar plastics sorting machine operating with a diffraction grating to spread the infrared spectrum over an output strip and a small number of sensors corresponding to wavelengths which are unevenly distributed in this output strip. It is mentioned in this document that 10 well-selected sensors rather than the 256 sensors of a conventional bank of photodiodes may suffice. However, each of these 10 sensors has an area equivalent to each sensor of a bank, in other words typically a rectangle of 30×250 μm2. A surface of this type gathers little light and limits the speed of analysis to 200 measurements per second. Therefore, a spectrometer of this type cannot analyse all the points of a high-speed conveyor with the above-mentioned speeds and resolutions.
This last document therefore proposes the production of a line of identical parallel microspectrometers for analysing a planar flow. According to the inventor, the cost of a spectrometer would be minimised by microsystem production techniques, but the necessary resolution involves 25 to 50 spectrometers on the line to cover the width of the conveyor mat: the total cost, like the maintenance constraints, are therefore very high. In addition, few details are provided in this document on the production of such a machine, and there does not seem to be any machine of this type currently in operation.
In addition to the drawbacks and limitations inherent in each of the above-mentioned devices and methods, one major drawback which is common to all these devices and methods should be mentioned, namely their inability to reliably process objects having a significant height, for example of about 10 to 30 cm, owing to the inadequate intensity of applied radiation at this distance from the plane of conveyance Pc of the travelling objects, or owing to the inability to recover the radiation to be analysed or else for both the aforementioned reasons.